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64 Hazardous Materials Transportation Incident Data for Root Cause Analysis eight is entered, the eight would be rejected; similarly, alphas would be rejected if the field were numeric. There are also extensive computer consistency checks for each field, in which the value in one field is compared with the values in other fields to ensure that the fields are consistent. For example, if the first harmful event is a collision with a non-motorist, the field for the number of non-motorists involved must not be zero. There are multiple consistency checks for each field. Some of the checks are prescriptive, that is, certain values must be registered, while others flag unusual situations that should be reviewed. The consistency checks are documented in the Cod- ing and Validation Manual (NHTSA 2002) for the FARS system. In addition, analysts receive annual training at a national meeting. Similar quality control procedures are implemented as the cases are aggregated into the final file. The records are reviewed for timeliness and completeness. Statistical control charts are used to monitor the coding of key data over time, to see if distributions are wandering according to past experience. Typically, three versions of the FARS file are released. The "early assessment" file is released as a partial file that provides an initial look at the data for a year. Next there is a "complete" version that is typically released in the fall of the year following the data year. A "final" version, which includes all corrections and additional cases that have accumulated, is sub- sequently released, typically 18 or more months after the data year. In some ways, the FARS file is the gold standard for data on fatal crashes. It is the product of considerable care over time, and is produced by a system that incorporates many checks for con- sistency and accuracy. On stable, well-understood data elements such as the environment of the crash, it is assumed to be of high quality and accurate. Items such as weather, time of day, light condition, and road type are coded from police reports and are dependent on the accuracy of the PAR. Although the accuracy of this information is unknown, these conditions are relatively sta- ble and should be identified on the PAR with acceptable accuracy. Similarly, it is assumed that the FARS file is acceptably complete (that is, virtually all vehicles involved in a fatal crash are included). However, given the sheer number of fatal crashes annu- ally (about 40,000) and vehicles involved (about 55,000), it is not possible that every crash and every vehicle involved is included. In the process of compiling the annual TIFA file, each year, a small number of trucks appear on a police report for a fatal crash, but the FARS file contains no record for that vehicle. It is also possible that some fatal crashes are missed because the fatality occurs toward the end of the 30-day window. These few omissions are, however, undoubtedly very small in number and inconsequential. The accuracy of the FARS data with respect to the main concerns of this project is a more com- plex matter. The TIFA file, indeed the entire TIFA protocol, allows an independent check for the accuracy of those data elements in common between it and the FARS file. The inconsistencies with respect to the hazmat variables have been noted above. There also are problems with the identification of trucks in FARS. Cases extracted for the TIFA survey include some categories of vehicles that are not identified as medium or heavy trucks in FARS. These include light vehicles coded with a GVWR of more than 10,000 pounds. Each year, the TIFA survey determines that 200 to 300 of these vehicles are, in fact, medium or heavy trucks. In addition, the TIFA survey determines that a number of the vehicles identified in FARS as trucks are actually light vehicles. Typically there are 60 to 70 vehicles identified in FARS as a medium or heavy truck that prove to be a light vehicle or something other than a truck. 4.3.7 Additional Fields The FARS system is quite complete and includes valuable fields. However, the addition of other data fields to the protocol would contribute to the ability to analyze crash causation. Some could be very easy to include, and require little modification of the program. Others would take